The invention relates to a method and an arrangement for determining a fuel quality of fuel for a combustion engine as well as to a computer program and a computer program product.
Biodiesel is a vegetable oil- or animal fat-based diesel fuel, made by chemically reacting alcohol with lipids from, by ways of example, rape seed oil, palm oil or soy oil, generating long-chain alkyl esters of methyl, propyl or ethyl. Biodiesel is used in standard diesel engines. In standard diesel engines equipped with injection pumps biodiesel can be used either unblended in pure form (“B100”) or, alternatively, may be blended with mineral diesel fuel (petroleum diesel, petrodiesel) at any concentration. However, for instance, new high pressure (29,000 psi=200 MPa) common-rail diesel engines have strict factory limits of B5 or B20 (meaning a permitted maximum of 5 vol % and 20 vol % of biodiesel in the blend) depending on manufacturer.
Biodiesel can be used as fuel without altering the combustion engine. However, biodiesel as well as synthetic diesel have densities and specific caloric values different from petrodiesel. This may result in the necessity to alter the fuel quantity for instance in an injection system for achieving the same engine torque and same rotational speed when using varying fuels of fuel compositions. Altering the injected fuel quantity on the other hand may require an adaptation of control parameters of the engine control as this may influence the exhaust temperature as well as the exhaust composition.
For instance, the amount of soot generated in the engine as well as the content of nitrogen oxides is influenced by the fuel composition and may even be improved compared to petrodiesel, so that the exhaust regeneration has to be adapted, too. Typically, the engine calibration is usually not optimally adapted with respect to emissions, noise, fuel consumption and the like when such synthetic diesel or biodiesel fuel additives are used together with petrodiesel or, if petrodiesel is completely replaced, by such alternative fuels.
EP 2 080 888 A2 discloses a method for determining the quality of the fuel for a direct-injection internal combustion engine in a vehicle. The common-rail fuel injection system comprises a fuel pump which is controlled by a PID-regulator. When the fuel viscosity changes the I-value of the PID-regulator will change. It will also change in response to the modified viscosity and thus the modified rail pressure. Subsequently, the new viscosity can be determined using the new I-value in combination with reference values stored in a characteristic map. The evaluation of the viscosity of the fuel is performed during a stationary or a degressive operation mode of the combustion engine.
DE 102011077404 relates to a method for determining fuel quality in conjunction with a high-pressure injector, by analysing a pressure increase curve.
WO 2009056402 relates to a method for identifying a fuel type, using measurements of the pressure in a high-pressure area over time.
EP 0 828 070 and EP 1 873 378 relate to methods involving characterization of a fuel type using a pressure signal.
DE 102 52 476 relates to a method involving determination of a type of fuel, utilizing an analysis of the movement of a magnetic valve.
W002/084101 relates to a method involving determination of a value significant for the energy content of a fuel, utilizing the movement of a movable valve element.
It is desirable to provide a method and an arrangement for determining a fuel quality as fast and as accurately as possible with existing hardware in the vehicle.
According to a first aspect of the invention, a method is proposed for determining a fuel quality of a fuel for a combustion engine, particularly for a diesel engine, wherein the fuel is conveyed from a low-pressure fuel tank to a high-pressure volume and injected into at least one cylinder of the combustion engine, wherein a control valve is provided for controlling directly or indirectly the amount of fuel injected into the at least one cylinder. An actual value of a timing signal of the control valve is compared to a reference value of the timing signal of the control valve and a fuel quality parameter is derived from a difference between the actual value and the reference value of the timing signal of the control valve.
Additionally or alternatively, a fuel quality parameter is derived from a gradient of the pressure increase during a build-up phase of the pressure in the high-pressure volume compared to a reference value of the gradient of the pressure increase in the high-pressure volume.
Favourably, the method can be employed in common-rail injection systems as well as in unit-injector systems of diesel engines.
Expediently, the control valve can be a spill valve between the fuel tank and the high pressure volume—which is the common rail in a common rail injection system or a pump chamber in a fuel pump of a unit-injector system. The fuel quality parameter may be derived from a difference between the actual value and the reference value of the timing signal of a spill valve of a fuel supply system and/or from a difference between the actual value of the pressure gradient and a reference value of the pressure gradient in the fuel system.
Expediently, the fuel may be conveyed from the fuel tank at a first pressure to the high-pressure volume at a pressure higher than the first pressure and injected into at least one cylinder of the diesel engine, wherein the spill valve is provided for limiting the fuel pressure in the high-pressure volume.
In a favourable embodiment, a required pressure in the high-pressure volume is established via a closed loop control of the pressure by adjusting the control valve timing and/or its operating duration.
Expediently, the actual value of the timing signal of the control valve may be determined at constant engine torque. The accuracy of the determination of the control valve timing is improved by stable operation conditions of the engine.
In a further expedient embodiment, the actual value of the timing signal of the control valve may be refined with an actual value of the temperature of the fuel. Temperature dependencies of fuel density and fuel viscosity can be eliminated, thus improving the accuracy of the inventive method.
In a further favourable embodiment, the actual value of the pressure signal of the high-pressure volume may be refined with an actual value of the temperature of the fuel. Temperature dependencies of fuel density and fuel viscosity can be eliminated, thus improving the accuracy of the inventive method.
In a further expedient embodiment, the reference value of the timing signal of the control valve may be derived from a set of characteristic curves for a reference fuel. The reference fuel may be petrodiesel the parameters of which are well known.
Determining the quality of the fuel the invention advantageously solves the problem of measuring the composition of the fuel when running on pure biodiesel or pure synthetic diesel or a blend of diesel and synthetic diesel or biodiesel (e.g. RME; RME being a short form for rape seed biodiesel). An advantage is that no additional hardware is needed in the case of common-rail injection systems. In the case of unit-injector systems, it is expedient to couple a pressures sensor to the high-pressure volume.
The installation of an additional fuel sensor can be avoided which will reduce the total engine cost. The timing signal of the control valve can be easily derived from the control signals of the control unit controlling the operation of the control valve which, for instance in a common-rail diesel engine, can be the control unit controlling the operation of the various actuators in the common-rail arrangement (also called the rail controller). The timing signal is preferably an on-signal or an off-signal for the control valve. The activation of the spill valve in the case of a common-rail injector system is programmed (i.e. mapped) for all engine operating conditions and thus available in the respective control units.
The invention is particularly usable in a common-rail injection system where fuel is conveyed from a low pressure fuel tank to a common rail (the high pressure volume) at a high pressure before being injected into the at least one cylinder of the combustion engine.
In a common-rail diesel engine, fuel is conveyed from a low pressure fuel tank to a high-pressure volume, i.e. a common rail, and injected into the engine via the common rail. The spill valve can be understood as a pressure control valve. Rail pressure and injection duration determines the amount of fuel injected.
For instance, in a common-rail injection system the fuel pressure is generated by a plunger (piston) pump in a high pressure reservoir (which is called the rail). The pressure within the rail is adjustable via a spill valve control. For a given rail pressure and a given amount of fuel injected into the (engine) cylinder, a certain duty cycle of plunger pumping is needed. The pumping period is represented by a parameter called “duty cycle” used in the control unit (ECU) of the rail (the rail controller). In order to obtain the required rail pressure, a pressure sensor is placed in the rail, and the adjustment of the correct pressure is controlled via a closed loop control of the rail pressure by adjusting the timing and/or duration of the operation of the spill valve (in the following also described as the spill valve timing and/or its operation duration). This is favourably used to detect differences in physical fuel properties, such as viscosity and/or density of the fuel. If these properties of the fuel vary, the pumping work has to decrease or increase compared to pure petrodiesel, i.e. the duty cycle has to be changed. For instance, if the values of the viscosity and density of the actual fuel other than petrodiesel are lower than those for petrodiesel, the pumping period has to be increased correspondingly for such other fuel, and, consequently, has to be decreased, if the values of the viscosity and density of the actual fuel are higher than those for petrodiesel. The respective petrodiesel properties are well known and standardized and, therefore, can be utilized advantageously as reference properties.
Alternatively or additionally, the actual value of the timing signal of the control valve may be refined with an actual value of the pumping period of a fuel pump conveying the fuel from the fuel tank to the high-pressure volume such as the rail (particularly in a common-rail injection system). The accuracy and robustness of the method can be improved by including more signals for the determination of the actual value of the timing signal of the control valve.
The method is particularly also usable in a unit-injector system where the fuel is compressed in a pump volume while pumping is taking place during injection of the fuel into the at least one cylinder. Particularly, in a unit-injector diesel engine fuel is injected by needle valves in individual cylinders of the engine. A spill valve is provided between a fuel pump and a fuel tank. In case of a unit-injector system, the spill valve timing is equivalent to a specific crank angle position. The fuel quality parameter can be derived from the spill valve timing as well as from a gradient of a pressure during a build-up phase versus a spill valve timing (i.e. a crank angle position) in a high-pressure volume, particularly a pump chamber, during injection of the fuel.
The fuel quality parameter is derived from a gradient of the pressure increase during a build-up phase in the high-pressure volume compared to a reference value of the pressure signal of the high-pressure volume, particularly a pump chamber of a fuel pump.
In case of the unit-injector system, the activation of the spill valve is programmed (i.e. mapped) for all engine operating conditions and thus available in the respective control units such as a spill valve control. There is no common rail in such a system. The fuel pump compresses the fuel in the pump chamber, which is the high-pressure volume in case of the unit-injector system, and the fuel is injected into individual cylinders by needle valves arranged at each of the individual cylinders. Injection takes place while the pump is pumping fuel. The needle valve controls the fuel injection into the combustion chamber, the injection timing and the injection duration. In prior art systems, the fuel pressure is mapped in a control unit and, consequently, the fuel pressure is not measured. According to the invention, by coupling a pressure sensor to the high-pressure volume, i.e. the pump chamber of the fuel pump, a pressure can be determined in the pump chamber.
Expediently, in the unit-injector system the required pressure in the high-pressure volume is established via a closed loop control by adjusting the needle-opening pressure of an injector valve and/or by adjusting the actual value of the timing signal of the control valve and/or its operating duration.
Determining the quality of the fuel the invention advantageously solves the problem of measuring the composition of the fuel when running on pure biodiesel or pure synthetic diesel or a blend of diesel and synthetic diesel or biodiesel (e.g. RME; RME being a short form for rape seed biodiesel). The installation of an additional fuel sensor can be avoided which will reduce the total engine cost.
The pressure signal of the high-pressure volume can be easily derived from a pressure sensor coupled to the pump chamber, e.g. in the pump chamber or close to it, and submitted to a control unit controlling the operation of the control valve. For instance, for a unit-injector system, the injector pressure will be higher if the engine is run on biodiesel compared to the engine run on petrodiesel and the pressure can thus be monitored.
In case the required maximum pressure for injection is fixed, the pressure in the pump chamber of the fuel pump will increase faster for biodiesel than for petrodiesel. This is favourably used to detect differences in physical fuel properties, such as viscosity and/or density of different fuels. If these properties vary, the gradient of the pressure increase in the build-up phase of the pressure in the pump chamber is lower or higher compared to pure petrodiesel. The respective petrodiesel properties are well known and standardized and, therefore, can be utilized advantageously as reference properties. In a conventional unit-injector system, a steeper increase of the pressure would result in a higher pressure which could damage the unit injector.
According to a further aspect of the invention, an arrangement is proposed for performing a method for determining a fuel quality of a fuel for a combustion engine, particularly of a diesel engine, where the fuel is conveyed from a low-pressure fuel tank to a high-pressure volume and injected into at least one cylinder of the combustion engine, wherein a control valve is provided for controlling directly or indirectly the amount of fuel injected into the at least one cylinder. A control unit is provided for comparing an actual value of a timing signal of the control valve to a reference value of the timing signal of the control valve and deriving a fuel quality parameter from a difference between the actual value and the reference value of the timing signal of the control valve.
Additionally or alternatively, a fuel quality parameter is derived from a gradient of the pressure increase during a build-up phase of the pressure in the high-pressure volume compared to a reference value of the gradient of the pressure increase in the high-pressure volume.
This arrangement may be particularly suited for diesel engines with a common-rail injection system, or with a unit-injector system where the control valve can be the spill valve of the system which spill valve is used to limit the pressure in the high pressure volume.
In a favourable embodiment, a required injection pressure may be established via a closed loop control of the pressure by adjusting the control valve timing and/or its operating duration. In case of a unit-injector diesel engine the needle valves associated with the individual cylinders control the fuel injection into the combustion chamber, the injection timing and the injection duration.
Alternatively or additionally, the actual value of the pressure signal may be refined with an actual value of the pumping period of a fuel pump conveying the fuel from the fuel tank to the high-pressure volume such as the fuel pump chamber in a unit-injector system. The accuracy and robustness of the method can be improved by including more signals for the determination of the actual value of the pressure signal of the high-pressure volume.
Expediently, a temperature sensor may be provided for determining an actual value of the temperature of the fuel, thus improving the measurement accuracy of the method. Further, a pressure sensor may be provided for measuring the pressure and/or a pressure gradient in the high-pressure volume.
According to another aspect of the invention, a vehicle comprising an arrangement for performing a method for determining a fuel quality of a fuel for a combustion engine, particularly a diesel engine, is proposed, where the fuel is conveyed from a low pressure fuel tank and injected into at least one cylinder of the combustion engine, wherein a control valve is provided for controlling directly or indirectly the amount of fuel injected into the at least one cylinder. The vehicle may be equipped with a common-rail injection system. In this case, an actual value of a timing signal of the spill valve is compared to a reference value of a timing signal of the control valve and a fuel quality parameter is derived from a difference between the actual value and the reference value of the timing signal of the control valve.
Alternatively, the vehicle may be equipped with a unit-injector system where differences of the spill valve timing compared to a timing related reference value and/or differences in the gradient of a pressure increase during a build-up phase in a fuel pump chamber compared to a pressure-gradient related reference value are used to derive the fuel quality.
According to another aspect of the invention, a computer program is proposed comprising a computer program code adapted to perform a method or for use in a method for performing a method for determining a fuel quality of a fuel for a combustion engine when said program is run on a programmable microcomputer, where the method includes conveying fuel from a fuel tank and injecting the fuel into at least one cylinder of the combustion engine, wherein a control valve is provided for controlling directly or indirectly the amount of fuel injected into the at least one cylinder and wherein, for instance in case of a common-rail injection system, an actual value of a timing signal of the control valve is compared to a reference value of the timing signal of the control valve and a fuel quality parameter is derived from a difference between the actual value and the reference value of the timing signal of the control valve. Additionally or alternatively, differences in the gradient of a pressure during a build-up phase in a high-pressure volume may be used to derive the fuel quality. The computer program may be adapted to be downloadable to a control unit or one of its components when run on a computer which is connected to the internet.
According to a further aspect of the invention, a computer program product stored on a computer readable medium is proposed, comprising a program code for use in a method as described above on a computer.